Massive structures of plants have been built on the basis of photosynthesis. In this study, the effects of different light intensity on the leaf photosynthesis, transpiration, CO2 uptake and stomatal conductance was examined. Gas exchange was measured in the planted forest of European ash tree (Fraxinus excelsior) in Hainich national park of Germany. Net photosynthesis rate (A), transpiration rate (E), CO2 stomatal conductance (Gs), intercellular CO2 concentration (Ci) etc. was measured for leaves of six ash individuals using LC-pro plus machine. The results showed that maximum net assimilation was about 7.6 kg CO2 per hectare per hour. The maximum intensity of the light used for photosynthesis in this study was 696 µmol m-2 s-1; regarding to the increasing rate of photosynthesis, it seems that the maximum rate of photosynthesis will occur at the PPF a little more than 696 µmol m-2 s-1. Transpiration rate and stomatal conductance of CO2 increased exponentially in all light intensities and their maximum were at 696 µmol m-2 s-1. The photosynthetic rate proved that current species is a semi sun plant (light compensation point = 10.6 µmol m-2 s-1), which can play the role of a shade plant in the forests of Central Europe (maximum intensity of photosynthesis = 3.5). This species has a valuable role in absorbing carbon in the mentioned forest ecosystem, which the annual carbon uptake equals to 1324 g m-2 year-1, which is about 2.5 times than the average uptake of all other species in the ecosystem.